Wireless communication systems generally include a cellular communications network and wireless devices (which may be referred to as user terminals or User Equipment devices (UEs)). The cellular communications network generally includes a Radio Access Network (RAN) including many base stations providing radio, or wireless, communications in corresponding coverage areas or cells. Wireless communications systems need means for the wireless devices to find transmissions of the base stations in the RAN of the cellular communications network. This is typically referred to as initial synchronization and is necessary when, e.g., a wireless device is powered on, loses a connection to the cellular communications network during a session, or when Radio Resource Management (RRM) measurements need to be made on neighboring cells.
In Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) cellular communications networks, signals referred to as a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS) are used for initial synchronization. The PSS and the SSS allow a coarse synchronization to a carrier transmitted by a base station for a particular cell. The PSS and the SSS are also used as a cell identification mechanism where 504 possible PSS/SSS sequences are spread across different cells. After coarse synchronization, the wireless device fine tunes its synchronization to the carrier using, in LTE, a cell-specific Common Reference Signal (CRS) transmitted in the carrier. Once the wireless device is synchronized to the carrier, the wireless device must then receive critical system information transmitted on the carrier by the base station such as, for example, a bandwidth of the carrier and other system parameters. For LTE, the system information is sent on a Physical Broadcast Channel (PBCH) with additional system information sent in System Information Blocks (SIBs) that are sent on a regular shared data channel used for packet data transmissions (i.e., Physical Downlink Shared Channel (PDSCH)).
In LTE, the PSS and the SSS are structured differently for Frequency Division Duplexed (FDD) and Time Division Duplexed (TDD) carriers. The PSS and the SSS are the same for FDD and TTD carriers, but the spacing between the PSS and the SSS for the FDD carrier is different than that of a TDD carrier. This allows early detection of the duplexing method used on a detected carrier. However, using different spacing between the PSS and the SSS for different carrier types increases initial search complexity.
Another issue with the conventional initial synchronization arises in the context of a heterogeneous deployment of a cellular communications network (i.e., a heterogeneous cellular communications network). Typically, as in the case in LTE, the synchronization signals used for initial synchronization are placed in a single location in the radio frame for a given duplexing mode. Therefore, detection of the synchronization signals allows the wireless device to determine the frame boundary of the radio frame on the carrier. This works well in a homogeneous cellular communications network where most of the base stations are transmitting with similar power. However, in synchronized heterogeneous cellular communications networks, the base stations (or radio access nodes) are transmitting at different power. As such, synchronization signals transmitted by higher power macro base stations can interfere with synchronization signals transmitted by lower power base stations (e.g., pico base stations).
Further, it is often desirable in a heterogeneous cellular communications network to have a wireless device connect to a low-power base station (e.g., a pico base station) even though downlink signals from the macro base station may be received with greater power. This is referred to as operating with a high Cell Selection Offset (CSO). Operating with a high CSO is typically done for two reasons. First, operating with a high CSO allows the load in a highly loaded network to be shifted from macro base stations with higher loading to low-power base stations, which typically have lower loads due to their small coverage regions. Second, operating with a high CSO allows uplink transmissions from wireless devices to be received at the low-power base stations when the received power at the low-power base stations is typically greater than that at the macro base stations.
When the wireless device connects to a low-power base station in spite of greater received power from a macro base station, the interference at the wireless device created by synchronization signals transmitted by the macro base station can be further exacerbated. For example, if the synchronization signals transmitted by the macro base station are received at a power of 8 decibels (dB) greater than the synchronization signals from the low-power base station, then the Signal-to-Interference Ratio (SIR) on the synchronization signals of the low-power base station would be −8 dB. Since the synchronization signals are static, this results in constant interference that can create problems for the wireless device when synchronizing to the low-power base station.
Another problem that occurs in wireless communication systems is carrier type detection. As described above, FDD and TDD carriers are differentiated in LTE by using different spacing between the PSS and the SSS. In future releases of LTE, a new carrier type may be defined for which legacy PSS/SSS sequences are to be used. If another set of spacings is used for the FDD and TDD modes of the new carrier type, this would result in increased complexity for the wireless device during initial search for PSS and SSS, which could already be a complex task. Thus, the same spacing between PSS and SSS is desirable from the point of view of wireless device complexity. However, if the same spacing is used between PSS and SSS for different carrier types/modes, detection of the carrier type/mode then becomes a problem.
There is a need for systems and methods that address the problems discussed above.